1. Field of the Invention
The present invention relates in general relays and, more particularly, to power system protective relays.
2. Description of the Related Art
The electrical power system in the United States generates three-phase alternating current (AC) electrical power. Each power phase is 120 degrees out of phase (plus or minus) with the other two power phases. The voltage of any phase oscillates sinusoidally between positive voltage and negative voltage. It is more efficient to transmit electrical power at high voltage levels than at low voltage levels. Electrical power is generated as three-phase AC power at moderate voltage and is stepped up to the 110 kV to 1000 kV range using a transformer for transmission over long transmission lines.
Switchgear is employed to control the transmission systems. The switchgear may comprise circuit breakers, fuses, switches, and relays. Electrical power distribution protective relays, referred to herein as relays, monitor a variety of electric power distribution parameters and control switchgear based on the state or condition of the electric power distribution parameters. For example, if too much current is being provided to a local distribution line, a relay may command a circuit breaker or switchgear to open, thus interrupting the supply of electrical power to that local distribution line. Current, voltage level, frequency, phase, and other parameters may be monitored by relays.
Relays vary in complexity from electromechanical devices monitoring a single parameter to microprocessor-controlled relays capable of monitoring many independent parameters concurrently. The relays are capable of changing their behavior when reprogrammed for monitoring various characteristics of a power distribution system. For safe and reliable operation of the power distribution system it is necessary to thoroughly test relays in the power distribution system at periodic intervals. Failures of the power distribution system can result in significant economic losses and inconvenience, and perhaps danger to people requiring special medical services dependent on electrical power distribution. For these reasons, periodic testing of the electric power system is mandated by government regulations. This periodic testing has been accomplished by connection of external test equipment to a relay for testing (i.e. Relay-Under-Test (RUT)). The external test equipment can be individual devices interconnected, or a consolidated test system or other external configurations. Significant testing effort is required by power companies and the like (i.e. medium voltage industrial plants), to confirm continued correct operation of relays.
Individual external test equipment 97 comprise a multitude of individual components used to perform testing, the equipment can include, for example, multimeters, clamp meters, portable appliance testers, oscilloscopes, spectrum analyzers, frequency counters, circuit breaker analyzers, signal generators and meters. The devices require special knowledge and training for interconnection, as well as connection to a relay subject to testing.
In addition to individual test components external to the relay, various test systems (i.e., special microprocessor based test devices in a single package (not shown)) have been used to test power system protective devices. With both electromechanical and electronic type relays a relay test system is temporarily connected with the relay trip unit circuit to test the response of the relay to various types of power system problems including overcurrent, undercurrent, line-line fault, line-ground faults, harmonic introductions, phase shifts, etc. Testing can also occur where the system monitors the relay's response to various input signals provided by the system. Some consolidated test equipment is commercially available, for example a single package Circuit Breaker and Overload Relay Test Sets, which are external to the relay and require correct connection thereto in order to perform properly.
Testing of relays has traditionally been accomplished through imitation of various input signals that the relay might be subject to, by emulating the input signals such as theoretical fault characteristics of a power distribution or generation system, thereby validating design elements associated with protection systems on a power distribution or generation system. Emulation can be considered an attempt to copy, as closely as possible, data from a real process (for example, data recorded by a device at a point in the power system). This emulation consists of various parameter characteristics associated with the power distribution system during a fault. Fault conditions (such as, current, voltage level, frequency, phase, and harmonics changes), generated through emulation, are therefore applied to the relay, and expected results are anticipated, validated or settings corrected for correct operation when the relay is applied in an actual power distribution system. Data imitation can also be performed by using current and voltage data recorded by a digital fault recorder located at a particular position in the power system; this data is only good for testing a relay that is located at the same point in the power system. Emulation data does not accurately represent actual power system values therefore the prior art test can be inaccurate and produce inaccurate results.
Much prior art relay test equipment 97 have multiple external signal generators to generate signals to test one or more relays. The systems include a controller to provide inputs for use by the signal generators to generate the signals. The systems can also include controller that could be hand held and include a user interface to communicate with the controller and to receive and display relay test information and to input control information for a user/test engineer.
In the prior art, one such package of external test equipment in essentially one enclosure is a relay test device 97 having a plurality of signal generators to generate signals to test one or more relays. The system includes a controller to provide inputs for use by the signal generators to generate the signals. The testing includes selecting to initiate a test of a relay by a user via an input of a hand-held controller (not shown) of the relay test device 97 and provides for transmitting an input signal to a control module of the relay test device 97 based on the user input, and initiating testing of the relay in response to the input signal. The testing also includes generating test signals by a generator of the relay test device 97 to test the relay, and capturing, by the control module, the results of the relay testing. The method also provides for displaying at least a portion of the relay testing information on a display of the hand-held controller.
Relays may be tested employing relay test equipment 97 with powerful power supply and amplifier and employing a separate personal computer (PC) to control the relay test equipment 97. The relay test equipment 97 may be relatively dumb and may depend upon the separate PC to provide the intelligence needed to test relays.
The use of various test equipment to attempt to produce or emulate theoretical fault conditions can be riddled with error sources such as incorrect settings, unmatched actual fault conditions, human error, etc. These errors cause inaccurate testing and hence affect the reliability of the electric power system. Personnel make mistakes in the process of isolating the relay and connecting the external test equipment. This can have a serious consequence for power system operation that is reviewed with respect to load loss and reliability. Additionally the cost of external test equipment and related training costs are very high.
Also, a relay test operator typically must have relatively advanced knowledge of relays and electrical power distribution to test the relays employing the relay test equipment 97. Personnel with advanced knowledge may be in limited supply, increasing the electrical power company's labor cost due to competition to hire limited skilled personnel or delaying testing due to shortages of skilled operators. In addition, the test operator must take special precautions to keep from accidentally tripping load from the electric power system and also to maintain a safe work environment. All of these precautions cumulatively can create a stressful work environment that influences the introduction of errors and accidents. Sometimes testing is carried out incorrectly, thus causing confusion regarding whether the Relay-Under-Test (RUT) is functioning correctly.